1. Field of the Invention
This invention pertains to marine seismic exploration and more specifically to a marine seismic streamer cable made up of identical sections each having a plurality of detectors or detector arrays which are selectably operationally connected and controlled to produce improved overall responses.
2. Description of the Prior Art
A marine seismic streamer cable is ordinarily towed behind a marine vessel for the purpose of gathering meaningful seismic data. The vessel, using suitable means, also includes or tows a device or devices for generating suitable source impulses into the water. The acoustic seismic responses therefrom are detected using suitable pressure detectors or hydrophones positioned along the cable. Such a cable, in addition to the detectors, also includes the communication wiring for conditioning the hydrophones for response and for transmitting the signals created at the detectors back to the vessel for suitable processing and/or recording. Moreover, such a cable includes the necessary water-proofing and other suitable components for minimizing spurious interference and water contamination from interferring with the meaningful signals. Finally, paravanes or other suitable means are also provided for vertically positioning the cable at the desired water depth along its length.
Historically, marine cables were initially made up of detectors positioned in a predetermined manner for the desired operation. This was done in either a continuous cable or in a cable made up of sections unique from each other where the last of the sections was the least complex in makeup. This was because it did not include the communication wiring except for the signals originating in that section, whereas the intermediate sections included wiring for carrying the signals originating with the earlier sections. Hence, the sections that were located in the intermediate locations along the cable length were progressively more complex from the end of the cable toward the vessel. The connections also were progressively more complex in the same manner, since they provided individual connections for the more signal wiring that existed closer to the vessel.
In addition to the cable structures becoming more complex, over the years the detectors themselves have become more complex. Originally, a single detector produced a single seismic signal from the pressure field impinging on the detector. Later, the detectors were arrayed so that typically seven detectors were employed to produce a single signal, the individual detectors in the array producing noise cancellation components from detected unwanted pressures and the individual signals were stacked or reinforced from the meaningful detected pressures of the individual detectors. That is, by connecting the detectors in arrays, the signal-to-noise response ratios from the detectors were enhanced.
The next significant improvement in cable development to note was the development of uniform sections and connections therefor. This development allowed a streamer cable to be made up of multiple sections with little attention paid to which section went where in the overall cable. That is, the sections were interchangeable. Not only did this development mean that maintenance and repair problems were minimized, but inventory problems were also minimized. Typically, a cable section of 50 meters in length comprised two sequential arrays, each 25 meters long. Each data channel was associated either with one of the arrays or the two sequential arrays in the cable section hard-wired together. The data channel thus developed was connected forward in the cable section on its own communications two-wire pair. As a communications channel two-wire pair progressed forward, one cable section at a time, the information was "rolled over" in the connectors or "cans" to a subsequent pair of wires. It may then be recognized that the sections near the end of the cable had many wire pairs that were not used since there were no preceding sections developing information channels. However, the sections and the cans were standardized. In similar fashion, cable sections of 100 meters in length having four sequential arrays were common and were similarly connected.
It was also possible to modify the connections of such a cable by making one array of double length out of two of the previous length arrays. This is because the spacing of individual detectors in an array was the same as the spacing between the two adjoining detectors of sequential arrays. It should be noted that the flexibility was limited in that the hard-wired connections were made for a particular array length, it was not possible to change the array length without re-hard-wiring. This was usually done by changing the connectors, each connector usually including a certain amount of internal wiring to make the optional hard-wiring connection selected.
Although the longer arrays did improve signal-to-noise ratios, the group interval (distance from the center of one array or group to the center of an adjoining array) was increased, which resulted in so-called "data smear" and lost resolution. One way which has been employed to overcome the difficulty of excessively long group interval has been to arrange the arrays so that they overlap. In such an arrangement, the interval can be shortened, with the group length being longer. Of course, to have an overlapping arrangement of detectors meant that the cable structure was made more complex than before, requiring more detectors in the same cable length and requiring means of electronically isolating the arrays from one another. This meant additional wiring and insulation means for the same cable length, compared with structures that preceded this development in the art.
One way of achieving overlapping arrays in cable sections of standard or universal makeup is shown in U.S. Pat. No. 3,441,902. One cable length comprises one group of detectors connected together in a primary string P. The same cable length includes two secondary strings S1 and S2. When connected together to adjoining cable sections, one complete array or group of detectors comprises an S1 string from a first cable section, a P string from a second cable section, and an S2 string from a third cable section. It should be noted that there is only one way to connect the arrays in such a system for the development of the respective composite group signals, and therefore there is no connection flexibility. Furthermore, the communication wiring is complex with three sections in the cable being involved for each array.
U.S. Pat. No. 3,441,902 also illustrates another recognized desirable optional detection phenomenon, that of non-uniformly spacing the detectors in an array to taper the response. As most clearly shown in FIG. 3 of this patent, the spacing of the detectors near the center of the array is much closer than the spacing of the detectors near the extremities of the array. This spacing is approximately in a statistical bell-shaped gaussian distribution pattern. In short, it has been discovered that it is desirable for some purposes to employ non-uniformly spaced detectors. Again, in the illustrated and described system revealed in U.S. Pat. No. 3,441,902, there is no flexibility of connection.
Therefore, it is a feature of the present invention to provide an improved marine seismic streamer cable employing universal sections, each having multiple arrays of detectors that are selectably recordable to result in flexible connections thereof.
It is another feature of the present invention to provide an improved marine seismic streamer cable comprising universal sections and universal connectors, the connectors having logic components that are remotely operable for permitting flexibility of electronic connection of the arrayed detectors included in the cable sections.
It is still another feature of the present invention to provide an improved marine seismic streamer cable employing, in a preferred embodiment, means for selectively connecting arrays of long group length in the cable sections such that the group intervals are kept short without a physical overlapping arrangement of transducers.
It is yet another feature of the present invention to provide an improved marine seismic streamer cable employing, in a preferred embodiment, means for selectively providing tapered or weighted detector response without varying the spatial positioning of the individual detectors.
It is still another feature of the present invention to provide an improved marine seismic streamer cable employing, in a preferred embodiment, means for providing dynamic spatial filtering to the acoustic signal as a function of time.
It is yet another feature of the present invention to provide an improved marine seismic streamer cable employing, in a preferred embodiment, means for providing spatial filtering for the acoustic signal as a function of distance from the acoustic source.